Java HashMap.size() – Examples

In this tutorial, we will learn about the Java HashMap.size() function, and learn how to use this function to get the number of items in this HashMap, with the help of examples.

size()

HashMap.size() returns the number of key-value mappings in this HashMap.

Syntax

The syntax of size() function is

size()

Returns

The function returns integer.

Example 1 – size()

In this example, we will initialize a HashMap hashMap with four mappings in it. To get the size of items in this HashMap, call size() method on this HashMap. Since there are four items in hashMap, size() should return 4.

Java Program

import java.util.HashMap;

public class Example{ 
	public static void main(String[] args) {
		HashMap<Integer, String> hashMap = new HashMap<>();
		hashMap.put(1, "A");
		hashMap.put(2, "B");
		hashMap.put(3, "C");
		hashMap.put(4, "D");
		System.out.println("HashMap  : " + hashMap);
		
		int size = hashMap.size();
		System.out.println("Size of this HashMap is : " + size);
	}
}

Output

HashMap  : {1=A, 2=B, 3=C, 4=D}
Size of this HashMap is : 4

Example 2 – size() – Empty HashMap

In this example, we will initialize an empty HashMap hashMap. To get the size of items in this HashMap, call size() method on this HashMap. Since there no items in hashMap, size() should return 0.

Java Program

import java.util.HashMap;

public class Example{ 
	public static void main(String[] args) {
		HashMap<Integer, String> hashMap = new HashMap<>();
		System.out.println("HashMap  : " + hashMap);
		
		int size = hashMap.size();
		System.out.println("Size of this HashMap is : " + size);
	}
}

Output

HashMap  : {}
Size of this HashMap is : 0

Example 3 – size() – Null HashMap

In this example, we will initialize a HashMap hashMap with null. When we call size() method on this HashMap, it should throw java.lang.NullPointerException.

Java Program

import java.util.HashMap;

public class Example{ 
	public static void main(String[] args) {
		HashMap<Integer, String> hashMap = null;
		System.out.println("HashMap  : " + hashMap);
		
		int size = hashMap.size();
		System.out.println("Size of this HashMap is : " + size);
	}
}

Output

HashMap  : null
Exception in thread "main" java.lang.NullPointerException
	at Example.main(Example.java:8)

Conclusion

In this Java Tutorial, we have learnt the syntax of Java HashMap.size() function, and also learnt how to use this function with the help of examples.